A keel of this type is known from GB 2 232 126 A.
The primary function of a keel in sail ships, especially in single-hull sailing yachts, is on the one hand to generate the transverse force in the water necessary for sailing by enlarging the lateral plane in order to create a reaction to the transverse wind force on the sail. On the other hand, keels are used in sail ships for holding ballast to increase the stability of the ship, since the transverse wind force applied to the sail generates a moment which heels the sail ship to leeward and which therefore must be partially balanced by a corresponding countermoment of the ballast migrating windward.
When the sail ship heels, the propulsive center of gravity in the sail acts alee, the center of gravity of resistance to propulsion of the entire wetted area of the underwater hull including the keel however lies windward. The interval between the resulting propulsive force in the sail and the resulting resistance force in the water generates a moment which makes the ship carry weather helm. In counter-steering, the rudder blade is oblique to the direction of travel and generates an additional flow resistance. Balancing by shifting the aerodynamic center of the sail to in front of the lateral center of gravity in the longitudinal axis of the ship does produce a countermoment in the direction of leeward-liness, but this moment due to its fixed lever length represents a given value, conversely the moment causing the windwardness has a different lever length depending on the heel. Therefore compensation of the two moments is not possible in all positions.
Another disadvantage of conventional keel designs is that the lateral plane, when the ship heels, is reduced by the sloped position of the keel fin so that the drift of the sail ship increases.
To avoid running aground in shallow water, especially in the vicinity of land or in harbor basins, either a long keel with shallow draft and large ballast is used, which greatly increases the total weight of the sail ship and thus also the wetted area of the hull, and thus causes increased resistance in the water. Bilge keels, keels swivelling to the rear or lifting keels are also known, the first two not having acceptable hydrodynamic properties. A lifting keel requires an extremely disruptive keel box which lies within the yacht.
Hydrodynamically most favorable are deep draft keels which are short in the longitudinal direction of the ship, therefore keels with a large extension and ideal profile cross sections. In addition to less ballast, because it is lower, and thus smaller wetted area with the same hydrostatic stability, these keels also have the lowest "induced resistance" as the largest resistance portion of known keel shapes. This resistance is caused by the pressure equalization around the lower edge of the keel from the pressurized lee side of the keel to the negative pressurized windward side of the keel, since the flow eddy which forms is extremely unfavorable. A reduction of induced resistance is only possible by partially suppressing the flow around the lower edge of the keel by wings. But they increase the wetted area and thus the frictional resistance of the underwater hull.
As the ship size increases however an appropriate draft becomes a problem, as already mentioned, due to channels. If the intention is to achieve the same efficiency of a keel of a 12 meter yacht with a draft of for example 2.35 meters in a yacht 30 meters long, it would have to have a keel depth of 6.35 meters.
As the size of the ship increases therefore at limited draft, to approach less deep harbors the conditions become more and more unfavorable. The extension becomes smaller and the ballast center of gravity is higher in relative terms. Stability decreases if balance is not created by a high ballast weight.